dovecot - Jul 2013 - Pigeonhole vacation auto-response not respecting "days" parameter

This might be a PEBKAC on my end, but in our old server, "vacation" 
responses would only be sent back to an origin address once per day 
because we had ":days 1" in the rule.

On the new server, even though we have ":days 1", pigeonhole is
sending
responses to every message, resulting in multiple vacation responses 
each day back to each origin address.

The following was generated by Roundcube's web interface, it looks to be 
correct.

-----------------------

require ["vacation"];
# rule:[vacation-reply]
if allof (not header :contains "Subject" "[spam]", not
exists
"List-Unsubscribe")
{
         vacation :days 1 :addresses 
["email1 at example.com","email2 at example.com"] :subject
"Out of office
reply" text:
I am currently out of the office July 11th and 12th, returning on July 15th.

During this time I will have no access to email.

Thank you.

User So-And-So
.
;
}

-----------------------

Interestingly, if I look at .dovecot.sieve.log, I see error messages 
like (except that the vacation responses are, in reality, being sent):

error: msgid=<20130711134816.35B9A40335 at mail.example.com>: failed to 
send vacation response to <originemail at example.com> (refer to server
log
for more information).

And in the maillog:

Jul 11 09:48:17 servername sendmail[28244]: r6BDmHem028244: 
SYSERR(UID1132): Who are you?
Jul 11 09:48:17 servername  sendmail[28244]: r6BDmHem028244: 
Authentication-Warning: servername .example.com: Unknown UID 1132 set 
sender to <> using -f
Jul 11 09:48:17 servername sendmail[28244]: r6BDmHem028244: from=<>, 
size=725, class=-60, nrcpts=1, 
msgid=<dovecot-sieve-1373550497-202242-0 at servername.example.com>, 
relay=Unknown UID 1132 at localhost

My guess at this point is that I've failed to configure pigeonhole 
and/or postfix's copy of the sendmail command properly so that it can 
create the "who I have send mail to recently" file.